Capacitive Micromachined Ultrasound Transducers (CMUTs) are increasingly being considered as a better alternative to traditional piezoelectric ultrasound transducers. In airborne applications, CMUTs offer the advantage of better impedance matching to the medium than piezoelectric transducers. One such application for CMUTs is in transit-time ultrasound flowmeters used for flare gas metering. Flare gas metering presents unique challenges due to the large variation in the flow velocities, gas pressures and gas composition. Ultrasound flowmeters are ideal for use in this application. However conventional CMUTs with vacuum backed plates cannot be used under widely varying ambient pressures. The pressure differential across the plate changes the static deflection of the plate, and as a result, the electric field through the gap. In a varying ambient pressure, the transmit and receive sensitivities and the operating frequency would vary considerably. Beyond a certain pressure, the CMUT plates would collapse onto the substrate and would drastically change their operating frequency.
In one attempt to address this problem, one group proposed operating CMUTs in a permanent contact mode even under 1 atm pressure. This would enable a more stable operating point over a wider operating pressure range. However, even such a CMUT would still be limited by the mechanical strength of the structure. Beyond a certain pressure, such a CMUT would fail mechanically.
What is needed is a CMUT that is capable of operating in environments ranging from relatively low pressure to several atmospheres of pressure.